The invention relates to an electric heating element, more particularly for a radiant heater body of an electric range made of a semiconducting ceramic, and to a method for its production.
DE 296 19 758 U1 discloses an electric radiant heater including an electric resistance heating element comprising a changing temperature coefficient of the electrical resistance. In a first temperature range extending from 0xc2x0 C. to at least 700xc2x0 C. the temperature coefficient is negative. In a second temperature range following the first temperature range the temperature coefficient is required to be positive. In this way it is intended that the resistance heating element automatically adapts to the second temperature range or to automatically limit any further increase in temperature above a critical value after an initial very fast increase in temperature to red hot.
The radiant heater takes the shape of at least one elongated rod or strip, its cross-section preferably being rectangular and is supported by its narrow edge on a substrate, the resistance heating element being configured solid.
The invention is based on the object of providing a heating element as cited at the outset permitting fast heatup and good regulation and comprising in general good heating properties, particularly advantageous values for the thermal surface loading or heating radiance, as well as providing a method for the production thereof.
This object is achieved by a heating element as it reads from the features of claims 1 and 21 as well as by a method as it reads from the features of claims 12 and 25. Advantageous aspects of the invention are the subject matter of the sub-claims.
In accordance with the invention the heating element may be made, on the one hand, of a semiconducting ceramic material which is open and/or porous at least in part or mostly and which more particularly in accordance with a first preferred embodiment may be foamy or spongy, particularly preferred cavitated. Foamy in this respect is to be understood as being a type of material in which the material comprises a large number of inclusions or chambers or pores preferably void. More particularly such a foamy ceramic material has the appearance of a sponge or a foam. Considered advantageous in this respect is a material having pores open to the environment to thus avoid gas release problems and the like.
Although the porosity may be selected over a wide range, a range of 10 to 50 ppi (pores per inch) is viewed advantageous, meaning that on a one inch line through the material 10 to 50 pores are cut or swept, corresponding roughly to 1 pore per mm. Values of approx. 30 ppi are considered to be particularly advantageous, thus making the material relatively finely porous.
A further advantageous aspect is the possibility of providing the ceramic material structured and limbed in the form of branchings which may result in, for example, a skeletonized structure in which the branching members are thin as compared to the cavitations. Advantageously the heat conducting material may be configured three-dimensionally meshed, more particularly similar to a three-dimensional textile material.
To cover both embodiments by one term the expression structured (foamed or skeletonized) ceramic is used in the following.
It is of advantage when in the course of the elongation of the heating element its electrically effective cross-section remains substantially the same to thus avoid in bent heating conductors the so-called hot paths of increased current flow, especially at the inner side of a curve. This is achieved to particular advantage by the skeletonized structure as described above, the limbs of which approach each other at such inner sides without changing their length or electrically effective conductor cross-section in each case.
The specific weight of the heating element or structured ceramic in both basic aspects may be in the range 0.1 to 3 g/cm3, preferably approx. 0.6 g/cm3 for 30 ppi for foamed ceramic, it being obvious from this that the proportion of pores or open sections exceeds the proportion of ceramic material by far, i.e. more pores or cavitations or interspaces existing than ceramic material. A solid ceramic material has a specific weight in the range 3 to 4 g/cm3. Thus the volume of the pores or cavitations or open sections may be partly ten to twenty times higher than that of the actual ceramic material. It is particularly to be noted that by employing a skeletonized ceramic the so-called porosity is even higher.
One significant advantage afforded by such a structured ceramic is that it exhibits a highly favorable ratio of conductor cross-section to radiant surface to thus permit the resulting heat to be emitted particularly well whilst achieving a very fast red hot heatup of the heating element. Advantageously a structured ceramic is configured elongated or rod-shaped.
The thermal surface loading is preferably approx. 12 W/cm2 at 1,200xc2x0 C. and approx. 16 W/cm2 at 1,300xc2x0 C., surface in this respect being, however, the envelope of the surface of the heating element, not the surface of the pure ceramic material.
The specific resistance can be in the range of approx. 0.25 Ohm*cm (cold) at around 30 ppi to approx. 0.4 Ohm*cm (at approx. 800xc2x0 C.). The value of the heating capacity can be set in one example embodiment with approx. 40% by weight silicon in the range of approx. 0.68 J/gK (cold) to approx. 1.15 J/gK (at approx. 1,000xc2x0 C.). Analogous to the ppi specification for the foamed structured ceramic it is the number of meshes per volume that dicates the cavitation size or density for the skeletonized version thereof.
The material of the heating element is preferably formulated with silicon, more particularly it may be formulated with silicon carbide, further alternatives being SiSiC, RbSiC as well as SiN whereby aluminum oxide, zirconium oxide or AlN may be used instead of silicon. A material formulated with silicon may also be MoSi2 commercially available as xe2x80x9ckanthal superxe2x80x9d, admixed to advantage with one of the aforementioned ceramics. Preferably the material of the heating element or this itself is sintered, The surface of the material may be coated with silicon oxide for surface protection, preferance being given particularly to silicon carbide doped with nitrogen or as an alternative reaction-bound silicon carbide to advantage. These procedures may take place to advantage in a reactive gas atmosphere.
The heating element may be formulated to advantage with Ti or TiN which is more particularly the electrically active material. The Ti material is coated on the outside to advantage with a protective coating which may be an oxide coating, for example SiO or Al2O3. Due to the mechanical properties of the compound or TiN it is preferably applied to a substrate, the material of which may be Al2O3 since this exhibits a similar thermal coefficient of expansion. A substrate or substrate matrix may be configured as described above, for example, skeletonized or foamed. As an alternative the heating element may be fabricated in a sandwich configuration in which the heating layer of TiN is applied to a substrate which is then covered by a protective coating. Such a sandwich-type heating element is configured preferably flat, for example as a flat rod incorporating a plurality of limbs, where necessary.
Another preferred alternative is to admix the TiN with matrix material, for example Al2O3, lending itself to good sintering. The specific electrical resistance of the mix depends on the volume percentage of the TiN which should, however, exceed 15%, although percentages as high as 50% or even 60% are still possible, also as regards workability. Such TiN admixed ceramic mixtures likewise require a protective coating, for example Al2O3.
Another alternative provides for a foamed or skeletonized structured material comprising a siliconized coating. One such structured ceramic, especially of SiC comprises a highly favorable conductor cross-section to surface ratio.
The heating element may be configured elongated, more particularly with at least one rod-shaped section extending, for example, transversely over a heating zone of a radiant heater body of an electric range, another alternative being a zig-zag or meander configuration of an elongated heating element for covering a larger surface area or bordering it. Alternative shapes of the heating element include a sheet configuration, for example in the form of a thin foil or the like.
For mechanical reinforcement the heating element may be fiber-reinforced, for which ceramic fibers, for example, are suitable and which may be inserted in the starting material prior to it being sintered into the ceramic.
The value for the specific weight of a structured ceramic may be selected less than approx. 5 g/kW down to approx. 1.7 g/kW to advantage.
It is via the porosity of the foamed ceramic and its pore size and number or mesh size of a skeletonized structured ceramic that the effective cross-section and/or the electrical resistance of the heating element can be set, the more or larger the pores or meshes the greater is the surface per mass unit and thus the radiance although the limiting factors in this respect are the ruggedness of and volume taken up by the heating element.
In addition the heating element may be treated, especially by doping or silicon infiltration, such that its temperature coefficient of the electrical resistance, particularly as viewed over the operating temperature range, does not change in sign. The operating temperature range may far exceed 1,000xc2x0 C., for example as high as 1,300xc2x0 C. or even max 1,600xc2x0 C. It is within this operating temperature range that the temperature coefficient should not change in sign to achieve a heating conductor which is defined with good control of the heatup characteristic. This may be, for example, a PTC characteristic, i.e. the electrical resistance increasing with increasing temperature, the heating element then automatically attentuating itself on heatup. In this arrangement the profile of the temperature coefficient may vary as a function of the temperature in each case, it not rising significantly until high temperatures are attained in avoiding overheating.
In the method in accordance with the invention for producing an electric heating element consisting of a semiconducting ceramic the starting material of the ceramic is admixed with a non-ceramic filler, the filler material being either insulating or incinerated in the sintering process to thus form the insulating interspaces, preferably cavitations, in the ceramic in subsequent sintering thereof.
In producing, for example, a foamed ceramic as described above the starting material is admixed with filler bodies having an insulating effect or other effect due to the change in temperature. The filler bodies are homogenously admixed with the starting material and may dissolve due to the thermal exposure in subsequent sintering in leaving the pores behind, it being in this way that the pores as cited above materialize as interspaces insulating the ceramic in sintering the starting material into the ceramic.
One material dissolving with increase in temperature is preferably a plastics material, for example, small balls of expanded polystyrene or the like. The size of the balls corresponds substantially to the desired pore size, their proportion the desired porosity.
Substantially the filler bodies can be admixed homogenously with the starting material, although it is also possible to add fewer or smaller filler bodies in configuring sections mechanically reinforced and/or thermally less stressed in these portions requiring a certain expense in filling the mold for the heating element. It is just as feasible, however, to compact the ceramic/filler mix in sections of less porosity with the addition of further ceramic material which is possible with no problem by elastic filler bodies (resulting in smaller pores).
Another alternative is to foam the ceramic starting material similar to the procedure in producing foamed plastics or the like. For this purpose a suitable binder can be admixed, such structures being finished with a subsequent coating of TiN and a protecting coating.
For producing a skeletonized structured ceramic as described above a textile material configured three-dimensionally and intermeshed may be impregnated with the fluid starting material for the ceramic. In this arrangement the starting material envelops the individual threads or skeleton limbs of the textile material in thus producing its structure. The textile material can thus form a kind of substrate for the ceramic. After impregnation the green body in which the starting material is preferably somewhat dried, is then baked causing the textile material to disappear or incinerate leaving the ceramic material behind, i.e. substantially in the form of the textile material including the branchings as a structured ceramic. The insulating interspaces correspond substantially to the mesh size of the textile material. Employed to advantage as the textile material is a fabric formed of knotted threads of considerable thickness or three-dimensionally or spatial extent. As an alternative several interconnected plies of a fabric may be employed. It is just as possible to make use of other forms of open substrates forming the pores or mesh. As a further example use may be made of an open pore foamed plastic in which the ceramic branchings exist less as skeletonized limbs but more in the form of of thin chamber walls or the like. Here too, a subsequent TiN or protecting coating may be applied.
Production may further include reforming the textile material which, on the one hand, may be done prior to impregnation with the ceramic material, or on the other preferably after impregnation in corresponding to the heating element as later desired.
For attenuating a temperature coefficient of the ceramic material the semiconducting ceramic may be doped with a dopant, use being made to advantage for this purpose, for example, of the porosity or openness for diffusing the gaseous dopant and accordingly for doping.
In the ceramic starting material, preferably existing fluid, a binder may be contained, more particularly to permit subsequent mixing with filler bodies or for wetting or impregnating the textile material.
As another alternative the ceramic after sintering may be subsequently annealed in an atmosphere containing the dopant. Doping may be done preferably with nitrogen which is diffused from the subsequent annealing atmosphere into the ceramic via the pores, the nitrogen absorbed by the ceramic being set via the duration of subsequent annealing or via the percentage of nitrogen in the atmosphere. When a ceramic is involved whose electrical conductivity or temperature coefficient can be influenced by the nitrogen absorbed, advantageous and desired properties of the ceramic can be set, this permitting in particular achieving an aforementioned temperature coefficient with no change in sign over the operating temperature range. Subsequent annealing may be done, for example, at temperatures of around 2,200xc2x0 C.
Furthermore in accordance with the invention an electric heating means including a heating element as described above may be provided, the heating means comprising a temperature sentinel assigned to the heating element including control means for influencing the heating element.
In accordance with the invention the heating element may further exhibit a negative temperature coefficient of its electrical resistance, this temperature coefficient being negative preferably over a broad temperature range covering or even exceeding, for example, the usual operating temperature of a heater. Since a temperature seriously exceeding the operating temperature of such a heater is to be avoided, where possible, it is the response substantially within this range that counts. The maximum operating temperature of the heating element lies way above 1,000xc2x0 C., for example 1,300xc2x0 C., preferably maximum 1,600xc2x0 C., more particularly just below the latter. Within the operating temperature range the temperature coefficient should not become positive in accordance with the invention, it preferably always remaining negative.
The advantage of the temperature coefficient being negative throughout lies not only in the faster increase to a red hot temperature and thus shorter heatup phase, since there is no xe2x80x9cdelayxe2x80x9d in attaining higher temperatures of the heating element, but also in the fact that although soft starting is provided, the current is then able to rapidly increase in prompting any further heating means cooperating with the heating element in accordance with the invention to produce a higher heat output. For regulating an xe2x80x9cultrafastxe2x80x9d heating element in accordance with the invention a temperature sentinel needs to be provided to prevent the heating element from exceeding the operating temperature range or a maximum temperature, the temperature sentinel being configured to advantage for ultrafast reaction. In all, this thus enables a heating means to be provided of ultrafast response and effect which due to the essential linear response of the heating element permits good regulation.
The material of the heating element is preferably formulated with silicon, more particularly it may be formulated with silicon carbide, further alternatives being SiSiC, RbSiC as well as SiN whereby aluminum oxide, zirconium oxide or AlN may be used instead of silicon. One siliconized material may also be MoSi2 commercially available as xe2x80x9ckanthal superxe2x80x9d. Preferably the material of the heating element or this itself is sintered. The surface of the material may be coated with silicon oxide for surface protection.
Preference is given particularly to silicon carbide doped with nitrogen or as an alternative reaction-bound silicon carbide to advantage.
A further preferred material is TiN as regards which reference is made to the comments above.
The heating element may be configured elongated, more particularly with at least one rod-shaped section extending, for example, transversely over a heating zone of a radiant heater body of an electric range, another alternative being a zig-zag or meander configuration of an elongated heating element for covering a larger surface area or bordering it.
Alternative shapes of the heating element include a sheet configuration, for example, in the form of a thin foil or the like.
For mechanical reinforcement the heating element may be fiber-reinforced, for which ceramic fibers, for example, are suitable and which may be inserted in the starting material prior to it being sintered into the ceramic.
The value for the surface loading of the heating element in one preferred embodiment is approx. 11.8 W/cm2 at around 1,200xc2x0 C. and approx. 16 W/cm2 at around 1,300xc2x0 C.
Also provided in accordance with the invention is a method of producing an electric heating element including an inherently negative temperature coefficient of the electrical resistance of the heating element, the heating element being configured more particularly in accordance with one of the alternatives as described above. In accordance with the invention the heating element is made of a semiconducting ceramic doped with a dopant to attenuate the negative temperature coefficient, nanoparticles being contained in the starting material permitting adjustment of the residual porosity of the ceramic after the starting material has been sintered. The residual porosity serves diffusion of a gaseous dopant and thus doping of the material with a dopant.
More particularly, the starting material can be compacted, preferably non-pressurized down to a relative density of 80% to 95%, more particularly approx. 90%. In the starting material which may be provided powdered a binder may already be contained. In subsequent sintering the porosity of the ceramic materializes due to the nanoparticles for which nanoscale carbon and/or submicron boron carbide may be admixed, for example, as sinter additives, the quantity of which thus permits adjusting the degree of porosity of the ceramic as regards both density and size of the pores.
After sintering the ceramic is preferably subsequently annealed in an atmosphere containing the dopant, preferably done with nitrogen. In this arrangement the dopant is able to diffuse from the atmosphere into the ceramic, more particularly into the existing pores. How much nitrogen is absorbed by the ceramic can be set via the duration of subsequent annealing. Since the amount of nitrogen absorbed by the ceramic influences the electric conductivity or temperature coefficient the advantageous properties as cited above as regards the temperature coefficient of the ceramic can be set. Subsequent annealing may take place for example at temperatures of around 2,200xc2x0 C.
Furthermore, an electric heating means including a heating element as described above can be provided, the heating means including a temperature sentinel assigned to the heating element with ceramic material for influencing the heating element. The heating means comprises preferably so-called radiant heater bodies under a cooktop, made of vitrified ceramic, for example.
These and further features read not only from the claims but also from the description and the drawings. Each of the individual features is achieved by itself or severally in the form of sub-combinations in one embodiment of the invention and in other fields and may represent advantageous aspects as well as being patentable in its own right, for which protection is sought in the present. Sectioning the application including sub-titling does not restrict the general validity of the comments made thereunder.